The present invention relates generally to wireless networks and more particularly to a system and method for multicast load balancing.
Multicast traffic is problematic in Institute of Electrical and Electronics Engineers (IEEE) 802.11 networks for a number of reasons. For example, high bandwidth multicast streams, such as video streams, can consume excessive bandwidth and potentially starve unicast streams. An IEEE 802.11 Basic Service Set (BSS) is the area covered by a single IEEE 802.11 access point. If an IEEE 802.11 Basic Service Set contains a single power-save client, i.e., a client capable of transitioning into a low-power mode as necessary, then all multicast or broadcast frames are sent in a multicast delivery period that follows a delivery traffic information message beacon. High priority unicast transmissions are stalled during the multicast delivery period. Thus, long multicast delivery periods have a significant adverse effect on IEEE 802.11 client power management. For example, a power-save client must stay awake for the entire multicast delivery period, even if the client only occasionally receives a broadcast address resolution protocol frame or other multicast frame.
One approach to this problem is to “rate limit” the total multicast traffic transmitted by an access point by configuring the maximum bandwidth used for multicast as a percentage of the total bandwidth. Under this approach, an access point simply discards new multicast frames whenever the percentage of bandwidth consumed for multicast traffic exceeds the configured maximum percentage. Such simple multicast rate limiting mechanisms are not sufficient, especially on low bandwidth access points, because simple rate limiting discards both useful and useless multicast frames.
Thus, there is a need for an alternative wherein an access point can use more fine grained, stream specific multicast rate limiting to support multicast streams well, rather than simple gross rate limiting of all streams.
In a wireless local area network, existing load balancing algorithms have been used to distribute wireless traffic across multiple access points in an overlapping coverage area as a function of the available bandwidth on each access point. In a typical load balancing implementation, access points advertise channel load information and clients migrate smoothly to the access point with the lightest load. The aggregate bandwidth in a Hot Spot area, e.g., a conference room, can be increased, for example, if multiple access points cover the Hot Spot area and the traffic load is “load balanced” across those access points. Load balancing can also be used to migrate clients from low bandwidth umbrella channels to high bandwidth Hot Spot access points whenever possible.
An IEEE 802.11 client's “load share” is that portion of the total load on the client's parent access point that is directly attributable to the client. A client's “unicast load share” is simply calculated as the total airtime used for frames sent to or from the client. It is much more difficult to calculate a client's “multicast load share.”
Load balancing algorithms are typically only effective in distributing the unicast traffic load across access points. For example, suppose a client is sending unicast frames and receiving both unicast and multicast frames. If the client roams from a first access point to a second access point, the client's unicast load shared is transferred from the first access point to the second access point. However, the multicast load on the first access point may not decrease and the multicast load on the second access may not increase after the client roams.
In some instances, load balancing can reduce the aggregate available bandwidth in a Hot Spot covered by multiple access points. For example, assume a conference room covered by at least two or more access points, and further assume that some clients must receive a high bandwidth Internet Protocol Television multicast stream. If the Internet Protocol Television clients are distributed across the access points using simple load balancing, then all of the access points must forward the Internet Protocol Television multicast steam. This greatly reduces the available bandwidth on all of the access points.
Thus, there is a need for a method for efficiently distributing high bandwidth clients, e.g., Internet Protocol Television clients, within a group of access points, thereby increasing the available bandwidth. Moreover, there is a need for a method for multicast load balancing in wireless networks.